1. Field of the Invention
The present invention relates to a developing device configured to develop an electrostatic latent image using a one-component developer (i.e., a toner). In addition, the present invention also relates to an image forming apparatus using the developing device.
2. Discussion of the Related Art
Methods for developing an electrostatic latent image are classified into two-component developing methods using a two-component developer including a magnetic carrier and a nonmagnetic toner, and one-component developing methods using a one-component developer including a toner and no carrier. Two-component developing methods have prevailed conventionally, but recently one-component developing methods are broadly used because of having advantages such that (1) it is not necessary to perform a carrier exchanging operation; (2) qualities of produced images can be maintained; and (3) a small and simple developing device can be used.
One-component developing methods use a one-component developer (hereinafter sometimes referred to as toner). The toner is frictionally charged with a developer layer thickness controlling member (hereinafter sometimes referred to as blade) and a developer bearing member (hereinafter sometimes referred to as developing roller) while a thin layer of the toner is formed on the developing roller. The toner on the developing roller is fed to a developing region in which the developing roller is opposed to an electrostatic latent image bearing member (such as photoreceptors) to develop an electrostatic latent image on the latent image bearing member, resulting in formation of a visual image (toner image) thereon.
The one-component developing methods include contact developing methods in which a developing roller is directly or indirectly contacted with an electrostatic latent image bearing member to develop an electrostatic latent image thereon. Specific examples of the indirect developing methods include jumping methods, methods utilizing an electric field curtain, and hopping development methods in which an alternate electric field is formed on the surface of a developer bearing member facing a photoreceptor so that toner particles hop, thereby forming a cloud of the toner, and an electrostatic latent image on the photoreceptor is developed with the toner cloud to form a toner image thereon.
For example, a published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 03-021967 discloses a hopping development technique such that an alternate voltage is applied to plural electrodes arranged at predetermined intervals on an insulating substrate to form an electric field curtain on the surface of a roller-shaped developer bearing member, thereby supplying the toner to the surface of a photoreceptor.
Since one-component developing methods have advantages in fine line image reproducibility and image density evenness, the methods are preferably used for forming high quality images. Therefore, various proposals concerning the hopping development technique have been made, for example, by JP-As 2007-133388, 2008-070674 and 2004-198675.
In the hopping development methods, a developer bearing member having an electrode array including plural small electrodes to which an alternate voltage is sequentially applied is typically used for forming an electric field curtain so that charged toner particles hop toward an image bearing member. In this regard, it is preferable for the electrode array thus formed on the developer bearing member to form the desired electric field even after long repeated use without causing errors due to adhesion of toner to the array. Therefore, JP-A 03-021967 proposes a technique in that an outermost insulating layer is formed on an electrode array, wherein the outermost layer is constituted of an insulating material such as synthetic resins (e.g., polycarbonate resins, nylon resins, fluorine-containing resins, polyacetal resins, phenolic resins, and polyethylene resins) and rubbers. However, this outermost insulating layer serves as a substrate for supporting a number of electrodes in such a manner that the electrodes are located at proper positions in the outermost insulating layer, and no cover layer is formed on the electrodes. In addition, since nylon, polyacetal and polyethylene resins and rubbers are relatively soft, toner for low temperature and high speed fixation use tends to easily adhere to the electrode array. In addition, it is described in JP-A 2007-133388 that using fluorine-containing resins for an outermost layer seriously decreases the charge quantity of toner in a short time.
JP-A 2007-133388 discloses the following:
(1) The surface of a developer bearing member should have such a high volume resistance as to prevent leakage of charge of toner from the surface of the developer bearing member;
(2) Since toner causes friction with the surface of a developer bearing member, and thereby a protective layer protecting the surface of the developer bearing member is frictionally charged so as to have a charge having the same polarity as that of the charge of the toner while charging the toner so as to have a charge having the opposite polarity, resulting in gradual decreasing of the charge quantity of the toner, the surface of the developer bearing member should have such a volume resistance as not to cause the toner charge decreasing problem;(3) When the surface of a developer bearing member has too high a volume resistance, the surface remains charged due to friction with the repeatedly hopping toner, resulting in variation of the surface potential of the developer bearing member, thereby making the development bias, which contributes to image development, unstable;(4) For the reasons mentioned above, the surface of a developer bearing member preferably has a volume resistivity of from 109 to 1012 Ω·cm, and it is preferable that the cover layer is made from a material selected from the group consisting of silicone resins, nylon resins, melamine resins, acrylic resins, polyvinyl alcohol resins, urethane resins, quaternary ammonium salts, and Nigrosine dyes; and(5) Two-component developers are relatively stable for high speed image formation compared to one-component developers.
It is not recommended in JP-A 2007-133388 to use a poly carbonate resin for the outermost layer.
JP-A 2008-076461 discloses techniques such that a layer having a thickness of from 0.5 μm to 2 μm and made of SiO2, BaTiO2, TiO2, or the like is formed as a cover layer of an electrode array using a sputtering method; or a layer prepared by forming a polyimide layer with a thickness of from 2 μm to 5 μm using a coating method, followed by baking, is formed as a cover layer, wherein when such a polyimide layer causes a problem, a layer is preferably formed on the polyimide layer, which has a thickness of from 0.1 μm to 0.5 μm and which is made of an inorganic material such as SiO2, by a sputtering method, and a layer may be formed on the inorganic material layer, which is made from an organic material such as polycarbonate resins, by a coating method. However, this protective layer manufacturing method is not simple because plural layers have to be formed by different methods.
On the other hand, heat fixing methods are typically used for fixing toner images formed by dry developing methods because of having good energy efficiency. Recently, the fixing temperature in heat fixing methods decreases more and more to save energy. Therefore, the present inventors recognized that in order to decrease the fixing temperature, it is necessary to use a low temperature fixable toner.
In contact one-component developing methods, toner is frictionally charged with a developer bearing member and developer a charging member (such as blades), while frictionally charged between a photoreceptor and a developer bearing member. Therefore, when such a low temperature fixable toner as mentioned above is used, a problem in that the toner is easily deteriorated by heat caused by the friction is caused.
Because of these reasons, the inventors recognized that there is a need for a one-component developing device, which can produce high quality toner images over a long period of time even when a low temperature fixable one-component developer (i.e., toner) is used.